Sulfite Reductase Hemoprotein (SiRHP) constitutes the catalytic subunit of the multicomponet enzyme complex Sulfite Reductase. Sulfite Reductase is responsible for the six electron reduction of sulfite to sulfide in the biological assimilation of sulfur. When supplied with artificial electron donors, SiRHP, a 64 KDa monomer, is fully capable of the reductive chemistry performed by the complex. The hemoprotein subunit utilizes a unique assembly of cofactors to transfer electrons to substrate. The structure of the oxidized enzyme is currently being built and refined. Atomic resolution information on this enzyme in its various complexed and reduced forms should lend great insight into the chemistry involved in the biological reduction of low molecular weight compounds.